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   FOUS11 KWBC 210754   
   QPFHSD   
      
   Probabilistic Heavy Snow and Icing Discussion   
   NWS Weather Prediction Center College Park MD   
   254 AM EST Sun Dec 21 2025   
      
   Valid 12Z Sun Dec 21 2025 - 12Z Wed Dec 24 2025   
      
      
   ...Upstate New York...   
   Day 1... Maximum impact level from WSSI: Minor   
      
   Clipper system racing across southeast Canada and a strong cold=20   
   front pushing across the Great Lakes and Northeast will provide the   
   chances for isolated snow squalls and a heavy lake effect snowband   
   downwind of Lake Ontario through tonight. Westerly flow around 40   
   kts at 850 mb and temperatures at this level around -15C will aid a   
   single snowband off Lake Ontario into Oswego and northern Oneida   
   counties. Flow bends towards a more northerly direction around 00Z   
   tonight before the band lifts back north on Monday. This wavering   
   will prevent significant snowfall accumulations, but moderate=20   
   amounts are still possible specifically prior to 00Z tonight. WPC=20   
   probabilities for at least 6 inches of snow are around 30-60% for=20   
   this region extending east from the southeast shores of Lake=20   
   Ontario.   
      
      
   ...Northeast...   
   Days 2-3... Probability of at least Minor Impacts: >60%   
      
   Upper level pattern over CONUS by late day 2 includes an anomalous   
   ridge centered over the central U.S. and strong jet soaring to the   
   north along the U.S-Canadian border before aiming southeastward   
   across the Northeast, where slight troughing remains. An embedded   
   shortwave crossing north of the Great Lakes on Tuesday within a=20   
   divergent left-exit region of a strengthening 150kt jet streak will   
   aid in blossoming precipitation across the Northeast by Tuesday.=20   
   Broad WAA ahead of the system over the still cold Northeast will=20   
   bring generally light snow to much of the Northeast and northern=20   
   Mid- Atlantic away from the immediate coast as early as Tuesday   
   morning. Some light freezing rain is also possible on the southern   
   edge of the precipitation (central PA and perhaps into parts of=20   
   WV) though the highest QPF will lie over Northern/Upstate/Central=20   
   NYS to southern New England. Snow is expected to track across New=20   
   England on day 3 as the shortwave crosses over the region and could   
   help spin up a surface low in the Gulf of Maine. This may lead to=20   
   an inverted trough or quickly developing TROWAL maximizing QPF=20   
   potential across southern Maine, but high uncertainty remains=20   
   within the day 3 timeframe. WPC probabilities for at least 4 inches   
   of snow are currently 20-40% from NYS through much of New England=20   
   (excluding regions south of I-90).=20   
      
      
   ...Cascades, Northern Rockies, & Northern California...=20   
   Days 1-3... Maximum impact level from WSSI: Major to Extreme   
      
   Digging heights near 135W in the eastern Pacific will help lift a   
   shortwave over the Northwest tonight as well as direct a resurgent   
   Atmospheric River into northern California with spillover=20   
   extending into the Northern Rockies. Snow levels will remain on the   
   lower side north of the moisture plume, but continued upslope into   
   the Cascades and Rockies will still yield modest snow totals for=20   
   these regions, especially days 1-2. The incoming shortwave tonight=20   
   will briefly raise snow levels in the WA Cascades from around 2000=20   
   to 3000ft, but then fall again with some CAA behind the system.=20   
   This will continue to impact the passes in the region (e.g.,=20   
   Snoqualmie and Stevens) where significant snow is likely. Heavy=20   
   snow in these regions of the Northwest and Northern Rockies may be=20   
   focused around two waves -- the initial one today/tonight with=20   
   another Monday night into early Tuesday. WPC probabilities for at=20   
   least 12 inches of snow for the three-day period are >50% above=20   
   3000ft or so.   
      
   Farther east, snow levels around 6000ft over the Northern Rockies=20   
   will rise today as heights rise and moisture increases. SW flow and   
   sufficient moisture will support lighter but still appreciable=20   
   snow compared to the Cascades over northern Idaho into=20   
   northwestern Montana, generally above 5000ft. The strongest=20   
   moisture flux will be in the core of the AR where PW and IVT=20   
   anomalies lie near/above the 99th percentile, generally in an arc=20   
   from the southern OR Cascades into the central/southern Idaho=20   
   ranges and into Wyoming. There, WSW to W flow will maximize=20   
   accumulations over parts of the Bitterroots and especially into the   
   Tetons, Absarokas, and Wind River Range. WPC probabilities for at=20   
   least 24 inches of snow over the three-day period are high (>70%)=20   
   over western Wyoming. High mountain peaks/ridges above=20   
   9000-10,000ft may see several feet of snow before precipitation   
   begins to wane on Tuesday.   
      
      
   *** Strong Atmospheric River to impact California and the Sierra   
    Nevada over the next several days ***   
      
   ...Sierra Nevada...   
   Days 1-3... Maximum impact level from WSSI: Major to Extreme   
      
   A long-lived Atmospheric River event will contain waves of=20   
   precipitation as embedded systems move inland from the Pacific over   
   the next several days (and beyond). An area of surface low=20   
   pressure will move inland across NorCal today and bring heavier=20   
   snow to the northern/central Sierra into early Monday with snow=20   
   levels around 8000-10,000ft from north to south. Snow may impact=20   
   some of the passes across the Sierra Crest though the heaviest snow   
   rates may be above 9000ft (2-3"/hr). Thereafter, the moisture=20   
   plume will lift northward and snow levels may lower a bit into=20   
   Tuesday with less accumulation overall except for the highest peaks   
   in the Shasta/Siskiyous and farthest north Sierra. The next surge   
   of moisture is expected by the end of day 3 as the lingering AR   
   plume orients more north-south in response to height falls just off   
   the West Coast. Snow levels also fall with this next round to   
   around 6000-8000ft from north to south. WPC probabilities for at   
   least 12 inches of snowfall are 50-90% above around 8000ft through   
   12z Wednesday. This very active and impactful weather pattern=20   
   across California is expected to continue through at least the next   
   several days, with additional high elevation heavy snow likely into   
   the end of this week. See our latest Key Messages that cover the=20   
   event through the end of next week, and CPC's Key Messages into=20   
   week 2.   
      
      
   The probability of significant icing is less than 10 percent.=20   
      
   Snell/Fracasso   
      
      
      
   ...Atmospheric River Key Messages are in effect. Please see=20   
   current Key Messages below...   
      
   https://urldefense.com/v3/__https://www.wpc.ncep.noaa.gov/key_messages/Late=   
   stKeyMessage_1.png__;!!DZ3fjg!_FA_POhDFb5HJN-sUJ0i9jl8edjRRHh_SXBYQveL5RWit=   
   qoz7Ca9u2HIoLQ1LcVdJwLI1W3MtAeUtLooha-m7g8eNWg$=20   
      
      
      
   $$   
      
   =3D =3D =3D   
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